Some display systems, for example liquid crystal displays (LCDs), are illuminated from behind. Such displays find widespread application in many devices such as laptop computers, hand-held calculators, digital watches, televisions and the like. Some backlit displays include a light source that is located to the side of the display, with a light guide positioned to guide the light from the light source to the back of the display panel. Other backlit displays, for example some LCD monitors and LCD televisions (LCD-TVs), are directly illuminated from behind using a number of light sources positioned behind the display panel. This latter arrangement is increasingly common with larger displays because the light power requirements, needed to achieve a certain level of display brightness, increase with the square of the display size, whereas the available real estate for locating light sources along the side of the display only increases linearly with display size. In addition, some display applications, such as LCD-TVs, require that the display be bright enough to be viewed from a greater distance than other applications. In addition, the viewing angle requirements for LCD-TVs are generally different from those for LCD monitors and hand-held devices.
Many LCD monitors and LCD-TVs are illuminated from behind by a number of cold cathode fluorescent lamps (CCFLs). These light sources are linear and stretch across the full width of the display, with the result that the back of the display is illuminated by a series of bright stripes separated by darker regions. Such an illumination profile is not desirable, and so a diffuser plate is typically used to smooth the illumination profile at the back of the LCD device.
A diffuse reflector is used behind the lamps to direct light towards the viewer, with the lamps being positioned between the reflector and the diffuser. The separation between the diffuse reflector and the diffuser is limited by the desired brightness uniformity of the light emitted from the diffuser. If the separation is too small, then the luminance becomes less uniform, thus spoiling the image viewed by the viewer. This comes about because there is insufficient space for the light to spread uniformly between the lamps.